Method and composition fos brighten-
ing aluminum



United States Patent 25,789 METHOD AND COMPOSiTlON FOR BRIGHTEN- ING ALUMINUM Marion D. Barnes, Glendale, Mo., assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware N0 Drawing. Original No. 3,094,489, dated June 18, 1963, Ser. No. 836,942, Aug. 31, 1959. Application for reissue Jan. 30, 1964, Ser. No. 353,310

9 Claims. (Cl. 252-79.2)

Matter enclosed in heavy brackets 1 appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention relates to the chemical brightening of metals, and is especially concerned with the chemical brightening of aluminum. The invention also relates to improved acid compositions suitable for use in chemical brightening of metals.

In recent years there has been a marked increasein the use of chemical brightening procedures for polishlng metallic surfaces. One of the more common processes used particularly in brightening metallic aluminum is carried out by treating the metal surfaces with a mixture of concentrated phosphoric and ni ric acids. That process is described in greater detail in Cohn, US. 2,729,551, the disclosure of which is hereby incorporated by reference into the present specification. It will be noted from the Cohn patent that compositions particularly suitable for brightening aluminum or its alloys generally contain between about and about 98 weight percent of free phosphoric acid, between about 0.5 and about weight percent of free nitric acid and between about 2 and about 35 weight percent of Water. Particularly preferred brightening compositions contain between about 65 and about 85 weight percent of tree phosphoric acid, between about i1 and about 10 weight percent of free nitric acid and between about 5 and about 34 weight percent of water. It might be added that after the compositions have been utilized for a period of time in the treatment of metals, significant concentrations of metallic phosphate salts will also build up in the treating compositions. For example, brightening compositions used for treating aluminum will generally contain between about 1 and about 20 weight percent of aluminum phosphate. The presence of such metal phosphate salts dissolved in the brightening compositions is generally not harmful, and in some instances may actually be helpful in so far as brightening of the metal surfaces is concerned.

In utilizing the foregoing acid compositions containing concentrated nitric acid, difficulty is often encountered because of :NO fumes released from such compositions. Such fumes are both disagreeable and toxic, and the presence thereof requires expensive precautionary measures and facilities for counteracting the resulting dangers. It has now been found that the evolution of N0 fumes from the compositions described above can be markedly reduced or substantially eliminated by the addition to such compositions of ammonia, ammonium hydroxide or an ammonium salt in concentrations such th at the ionic ratio of ammonium ion-s to nitrate ions in the compositions is maintained at or above 0.8, and preferably above about 1.1 or even above 1.3.

The exact mechanism whereby the addition of ammonia, ammonium hydroxide or ammonium salts prevents the evolution of N0 fumes is not completely understood, but it is bound that most of the nitrogen evolving from the compositions is in the form of N instead of N0 Surprisingly enough, it appears that the effect or the ammonium compounds is catalytic, since there is no loss or decrease in the iamonium ion concentration accompany- Re. 25,789 Reissued June 1, 1965 ing the conversion of N0 to N Because of this unexpected lack of consumption of ammonium ion, it is possible to maintain the ammonium ion concentration at a constant and controlled level by adding the ammonia or ammonium compound in a fixed ratio along with the phosphoric acid. .For this reason he addition of an ammonium compound as a solution in the phosphoric acid constitutes a preferred embodiment of this invention-as does also the phorphoric acid solution to which the critical concentration of ammonium compound has been added.

This latter embodiment of the invention comprises concentrated aqueous solutions of at least weight percent H P=O and preferably at least weight percent of H 90 to which between about .25 and about 4 weight percent, and preferably between about 0.5 and about 2 weight percent, of ammonium ion (based upon the total composition) has been added in the form of ammonia or an ammonium compound. Preferred compounds for such addition are inorganic ammonium salts, and particularly ammonium phosphate salts such as diamm-onium or-thophosphate. However, any other ammonium salts, such as ammonium sulphate, ammonium bisulphate, ammonium nitrate, monoammonium orthophosphat-e, ammonium metaphosphate, ammonium chloride, ammonium fluoride, ammonium fluoborate, ammonium ar-senat'e,

ammonium vanadate, as well as numerous others, willserve equally well in reducing or eliminating N0 fume evolution from the concentrated mixtures of phosphoric and nitric acids. Typical specific examples of the foregoing prefierred phosphoric acid/ammonium salt compositions are solutions prepared by:

(a) Adding 7 parts by Weight of mono-ammonium orthophosphate to 93 parts by Weight of 75% phosphoric acid (which will then contain 1.09 weight percent of ammonium ion);

(b) Adding about 414 parts by weight of diammonium orthop'hosphate to about 95.6 parts by weight of 80% phosphoric acid (which will then contain about 1.2 weight percent of ammonium ions);

(-0) Adding about 5 parts by weight of dia-mmonium orthophosphate to about 95 parts by weight of phosphoric acid (which will then contain about 1.36 weight percent of ammonium ions).

The obvious benefits resulting from the use of composi tions such as the foregoing are illustrated by the follow mg comparlson:

An aluminum brightening operation was carried on by dipping metallic aluminum objects for about 2 minute into .a chemical brightening composition containing abou 3 weight percent HNO about 76 weight percent H PO about 4 weight per-cent of AlPO and about 17 weigh percent of H 0 at a temperature of about C, Oopiou quantities of N0 fumes were evolved from the treatin bath, particularly whenever the metallic aluminum ob jects were immersed in the bath. The same operatic was carried out after incorporating about 4 weight per cent (based upon the total bath composition) of d ammonium orthophosphate. After the addition of the an monium salt there was no further evolution of NC fumes. There was also a significant reduction in but nitric acid and phosphoric acid consumption, but n noticeable decrease in effectiveness of the composition 1 a brightening bath.

Because of the so-ca-lled drag out losses resultir from liquid adhering to treated metal parts, it is nece sary to replenish the bath components from time to tim -In the operation described in the preceding paragra replenishment of ammonium compound and phosphor acid is conveniently effected by adding these two ingred cuts as a solution prepared by dissolving 4.2 parts I 3 weight of diammoni-um orthophosphate to 95.8 parts by weight of 85% H PO What is claimed is:

.1. A liquid concentrated phosphoric acid useful, when intermixed with from .about 0.5 to about 50 weight percent, based on the weight of said concentrated phosphoric acid, of nitric acid, for inhibiting the normal tendency of the resulting composition to evolve nitric oxide fumes when the ionic ratio of NH.;*"/ N in said resulting composition is at least about 0.8, brightening surfaces of metallic aluminum and aluminum alloys; said concentrated phosphoric acid containing less than about [125] 15 weight percent of water, and said acid having intermixed therewith a material selected from the and when it is utilized for 1 cent of free nitric acid, between about 2 weight percent and about 35 weight per-cent of water, and with which composition has been intermixed an ammonium salt in a sufiicient concentration to make the ionic ratio of NH4+/'NO3 equal to at least 0.8.

'6. The acid composition of claim 5 in which the ammonium salt is an ammonium oithophospha'te.

7. In the method of treating the suntaces of metallic aluminum and aluminum alloys with an acidic mixture containing between about 45 and about 98 weight percent of free phosphoric acid, between about 0.5 and about 50 weight percent of free nitric acid, and between about 2 and about 35 weight percent of water, the improvement which comprises incorporating into said acidic mixture 9.

group consisting of ammonia, ammonium hydroxide and material selected from the group consisting af ammonia,

ammonium salts in an amount suflicient to produce an ammonium ion concentration between about 0.25 and about [4] 2 weight percent.

2. A liquid concentrated phosphoric acid useful, when intermixed with from about 0.5 to about 50 weight percent, based on the weight of said concentrated phosphoric acid, of nitric acid, for inhibiting the normal tendency of the resulting composition to evolve nitric oxide fumes when the ionic ratio of NHJ/NO; in said resulting composition is at least about 0.8, and when it is utilized for 5 brightening surfaces of metallic aluminum and aluminum alloys; said concentrated phosphoric acid containing less than about 15 weight percent of water, and said acid having intermixed therewith an ammonium phosphate in an amount suflicient to produce on concentration between about 0:25 and about [4] 2 veight percent.

3. A liquid concentrated phosphoric acid useful, when nternuxed with from about 0.5 to about 50 weight per- :ent, based on the weight of said concentrated phosphoric .cid, of nitric acid, tor inhibiting the normal tendency if the resulting composition to evolve nitric oxide fumes rhen the ionic ratio of NHJ/NO said resulting comosition is at least about 0.8, and when it is utilized for 20 faces of metallic aluminum and an ammonum 30 in a d acid ammonium hydroxide and ammonium salts in an amount :suflicien't to maintain the ionic ratio of NH +/NO equal to at least 0.8.

'8. An acid composition suitable for brightening suraluminum alloys, which acid composition comprises at least about 45 weight percent of phosphoric acid and nitric acid, the ratio of said nitric acid to said phosphoric acid in said acid composition being such that said acid composition has a natural tendency when employed at elevated temperatures to release nitric oxide fumes, and an amount of a compound capable of releasing ammonium ions which is sufiicient to suppress said tendency of said acid composition .to release nitric oxide fumes, the ionic ratio of NHfl/NO;

composition being at least about 0.8.

9. An acid composition suitable for brightening surfaces of metallic aluminum and aluminum alloys, which acid composition comprises between about 65 and 85 weight percent of firee phosphoric acid, between about 1 and about 10 weight percent of nitric acid, the ratio of said nitric acid to said phosphoric acid in said acid composition being such that said acid composition has a natural tendency when employed at elevated temperatures to release nitric oxide fumes, and between about 5 and tightening surfaces of metallic aluminum and aluminum 40 about 34 Weight Piimfint Of Water, With Which acid lloys; said concentrated phosphoric acid containing less Ian about 15 weight percent of water, and said acid havlg intermixed therewith diammonium orthophosphate in n amount sufiicient to produce an ammonium ion con- :ntration between about 0.25 and :nt.

4. A liquid concentrated phosphoric acid useful, when termixed with from about 0.5 to about 50 weight per- :nt, based on the weight of said concentrated phosphoric :id, of nitric acid, for inhibiting "the normal tendency a the resulting composition to evolve nitric oxide fumes hen the ionic ratio of NHfi/NO; in said resulting m-position is at least about 0.8, and when it is utilized r brightening surfaces of metallic aluminum and aluinum alloys; said concentrated phosphoric acid containless than about 20 weight percent of water, and said id having intermixed therewith ammonium nitrate in amount sufficient to produce an ammonium ion concen vtion between about 0.25 and about 4 weight percent.

5. An acid composition suitable for brightening sur- :es of metallic aluminum and aluminum alloys, which nposition comprises between about weight percent 1 about 98 weight percent of free phosphoric acid, be- :en about 0.5 weight percent and about Weight perposition has been intermixed a material selected from the group consisting of ammonia, ammonium hydroxide and ammonium salts in an amount sufficient to maintain the ionic ratio of NHfl/NO in said acid composition equal about 2 Weight 45 to at least 0.8 in order to substantially reduce said tendency of said acid composition to release nitric oxide fumes.

References Qited by the Examiner The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,446,060 7/48 Pray et al 25279.2 XR 2,620,265 12/52 Hesch 25279.2 X 2,729,551 1/ 56 Colin 25279.=2 XR 2,746,849 5/56 Helling et al 25279.2 XR

OTHER REFERENCES JULIUS GREENWAL-D, Primary Examiner, 

